1. Field of the Invention
The present invention relates to an image forming apparatus and, more particularly, to an image forming apparatus which achieves an improvement in transfer performance by regulating the interfacial tensions of a photosensitive body, intermediate transfer body, transfer medium, primary transfer means, and secondary primary means with respect to pure water.
2. Description of the Prior Art
In conventional image forming apparatuses using an electrophotographic method, transfer operation is facilitated and a poor transfer phenomenon, in particular, such a phenomenon as a part of character or line is left out due to toner cohesion is reduced by, for example, a method of electrostatically facilitating transfer operation by regulating the relationship in surface energy between a photosensitive body and a transfer medium, a photosensitive body and an intermediate transfer body, and an intermediate transfer body and a transfer medium or regulating the relationship in contact angle with respect to pure water, and a method of making it easy for toner to move in a transfer direction even in the event of toner cohesion due to the pressure exerted by a transfer section (see Japanese Unexamined Patent Publication No. 08-211755, U.S. Pat. No. 5,732,314, and Japanese Unexamined Patent Publication No. 07-152263).
In each of these methods, the relationships in contact angle, surface energy, and surface tension between a member holding toner and a member on which the toner is to be transferred are regulated. However, there is no description about temporal values or changes over time.
Various materials are used for a photosensitive body, intermediate transfer body, and transfer medium used in an electrophotographic apparatus. Such a component is rarely made of a single material. In general, ozone or nitrate adheres to a photosensitive body serving as an image carrier because it is electrostatically charged. In addition, as toner and carrier are used in a developing process, toner components or external additive components adhere to the photosensitive body. In a transfer process, a transfer medium such as a paper sheet comes into direct contact with a photosensitive body or intermediate transfer body serving as an image carrier, and hence calcium carbonate, talc, or the like which is a paper component adheres to the image carrier.
As is obvious, surface roughness has influence on the surface energy of a photosensitive body or intermediate transfer body serving as an image carrier, the surface energy of a paper sheet as a transfer medium, and a contact angle with respect to pure water which is used to obtain such surface energy. The present inventors have found that the contact angle is greatly influenced by the surface state of a target member. In other words, the present inventors have found that the contact angle is greatly influenced by a substance adhering to the surface. In particular, the contact angle greatly changes depending on the time elapsed after pure water is applied. When a substance adheres to a target member, the contact angle greatly changes. The relationship in contact angle between a member holding toner or the like and a member on which the toner is to be transferred is reversed when changes over time are neglected.
The present invention has been made to solve the above problems in the prior art, and has as its object to provide an image forming apparatus which exhibits high transfer performance and forms high-quality images by regulating the surface physical properties of a member holding toner and a member on which the toner is to be transferred, and more specifically, contact angles, surface energies, and surface tensions, in consideration of changes in them over time.
In order to achieve the above object, according to the first aspect of the present invention, there is provided an image forming apparatus, comprising: transfer means to transfer a toner image formed on the photosensitive body surface onto one surface of the transfer medium, characterized in that when a time of passage through a contact transfer area between the photosensitive body and the transfer medium in a process of transferring the toner image from the photosensitive body onto the transfer medium is T sec, a contact angle of the photosensitive body with respect to pure water applied to the photosensitive body surface after a lapse of T sec since application of the pure water is larger than a contact angle of the transfer medium with respect to the pure water after a lapse of the T sec.
According to the second aspect of the present invention, there is provided an image forming apparatus described in the first aspect, wherein the transfer means comprises primary transfer means for bringing one surface of a transfer medium into contact with a photosensitive body surface and pressing the surface of the transfer medium against the photosensitive body surface.
According to the third aspect of the present invention, there is provided an image forming apparatus described in the first aspect, wherein the transfer means comprises primary transfer means for bringing one surface of an intermediate transfer body into contact with a photosensitive body surface and pressing the surface of the intermediate transfer body against the photosensitive body surface to transfer a toner image formed on the photosensitive body surface onto one surface of the intermediate transfer body, and secondary transfer means for bringing a transfer surface of the intermediate transfer body and one surface of a transfer medium into contact with each other and compressing the transfer surface and one surface of the transfer medium against each other to transfer the toner image formed on the transfer surface of the intermediate transfer body onto one surface of the transfer medium, wherein when a time of passage through a contact transfer area between the photosensitive body and the intermediate transfer body in a process of transfer operation from the photosensitive body to the intermediate transfer body is T1 sec, a contact angle of the photosensitive body with respect to pure water applied to the photosensitive body surface after a lapse of T1 sec since application of the pure water is larger than a contact angle of the intermediate transfer body with respect to the pure water after a lapse of the T1 sec, and when a time of passage through a contact transfer area between the intermediate transfer body and the transfer medium in a process of transfer operation from the intermediate transfer body to the transfer medium is T2 sec, a contact angle of the intermediate transfer body with respect to pure water applied to a surface of the intermediate transfer body after a lapse of T2 sec since application of the pure water is larger than a contact angle of the transfer medium with respect to the pure water after a lapse of the T2 sec.
According to the fourth aspect of the present invention, there is provided an image forming apparatus described in the third aspect, characterized in that when a time of passage through a contact transfer area between the intermediate transfer body and the transfer medium in a process of transfer operation from the intermediate transfer body to the transfer medium is T sec, a contact angle of the intermediate transfer body with respect to pure water applied to a surface of the intermediate transfer body after a lapse of T sec since application of the pure water is larger than a contact angle of the secondary transfer means with respect to the pure water after a lapse of the T sec.
According to the fifth aspect of the present invention, there is provided an image forming apparatus described in one of the first to fourth aspects, wherein contact angle adjustment for pure water in the time of passage through the contact transfer area is performed by applying a lubricant.
According to the sixth aspect of the present invention, there is provided an image forming apparatus described in one of the first to fourth aspects, wherein contact angle adjustment for pure water in the time of passage through the contact transfer area is performed by adjustment of surface roughness.
According to the seventh aspect of the present invention, there is provided an image forming apparatus described in one of the first to fourth aspects, wherein contact angle adjustment for pure water in the time of passage through the contact transfer area is performed by wettability adjustment based on plasma discharge.
According to the eighth aspect of the present invention, there is provided an image forming apparatus described in the first aspect, wherein transfer rollers formed from an elastic member are used as the transfer means.
According to the ninth aspect of the present invention, there is provided an image forming apparatus described in one of the first to third aspects, wherein a constant current source which outputs a constant current of 1 to 200 xcexcA with a polarity opposite to that of charged toner is used as the transfer means.
According to the tenth aspect of the present invention, there is provided an image forming apparatus described in the eighth aspect, wherein the transfer roller has a real resistance of 1xc3x97105 to 1xc3x971010xcexa9.
According to the eleventh aspect of the present invention, there is provided an image forming apparatus described in the first aspect, wherein the toner has a number average particle diameter of 3 to 8 xcexcm.
According to the twelfth aspect of the present invention, there is provided an image forming apparatus described in one of the first to eleventh aspects, wherein the photosensitive body has a drum-like shape.
According to the thirteenth aspect of the present invention, there is provided an image forming apparatus described in one of the third aspect, wherein the secondary transfer means for the transfer medium comprises an intermediate transfer body and a backup roller and secondary transfer roller which sandwich the intermediate transfer body, and performs constant current control, with the backup roller having a resistance of 1xc3x971015 to 1xc3x97107xcexa9, and the secondary transfer roller having a resistance of 1xc3x97105 to 1xc3x97107xcexa9.
According to the fourteenth aspect of the present invention, there is provided an image forming apparatus described in the ninth or eleventh aspect, wherein a coefficient of variation for a shape factor of the toner is not more than 16%, and a number coefficient of variation in a number particle size distribution is not more than 27%.
According to the fifteenth aspect of the present invention, there is provided an image forming apparatus described in the third aspect, wherein a relative position of the primary transfer means is within 10 mm on upstream and downstream sides of the central point of a contact transfer area between the photosensitive body and the intermediate transfer body in a rotational direction.
According to the sixteenth aspect of the present invention, there is provided an image forming apparatus described in the third aspect, wherein the primary transfer means is located downstream of the central point of a contact transfer area between the intermediate transfer body and the photosensitive body in a traveling direction of the intermediate transfer body.
According to the seventeenth aspect of the present invention, there is provided an image forming apparatus described in the third aspect, wherein a relative position of a secondary transfer roller serving as the secondary transfer means for performing transfer operation from the intermediate transfer body to the transfer medium is within 20 mm on upstream and downstream sides of the central point of a contact area between the intermediate transfer body and a backup roller in a rotational direction.
According to the eighteenth aspect of the present invention, there is provided an image forming apparatus described in the third aspect, wherein a nip forming roller which forms a nip between the photosensitive body and the intermediate transfer body is movable and is released except for image forming operation.
According to the nineteenth aspect of the present invention, there is provided an image forming apparatus described in the eighteenth aspect, wherein the nip forming roller is made of a metal and has no driving means for rotation.
According to the twenty aspect of the present invention, there is provided an image forming apparatus described in the third aspect, wherein the intermediate transfer body is placed in correspondence with a plurality of photosensitive bodies and developing units.
According to the twenty-one aspect of the present invention, there is provided an image forming apparatus described in the twenty aspect, wherein toner images are primarily transferred from the plurality of photosensitive bodies to one intermediate transfer body, and the toner images on the intermediate transfer body are secondarily transferred onto a transfer medium together.
According to the twenty-two aspect of the present invention, there is provided an image forming apparatus described in any one of the first, eleventh and fourteenth aspects, wherein a two-component developing agent constituted by toner and a carrier is used as the toner.
As is obvious from the above aspects, according to the present invention, the relationships in surface physical properties, and more specifically, contact angle, surface energy, and surface tension, between a member holding toner, such as a photosensitive body or intermediate transfer body, and a member on which the toner is to be transferred, such as an intermediate transfer body or transfer medium, are clarified in consideration of changes over time, and an image forming apparatus can be provided, which exhibits high transfer performance and forms high-quality images without causing any trouble such as poor transfer by regulating the physical properties.
The above and many other objects, features and advantages of the present invention will become manifest to those skilled in the art upon making reference to the following detailed description and accompanying drawings in which preferred embodiments incorporating the principle of the invention are shown by way of illustrative examples.